Isoguanine Poisoning. Purine is rapidly concentrated in the yeast cell, crystals of unassimilable isoquanine accumulate in the nuclear vacuole, rupture the membrane, and destroy the cell. Ascorbic acid in the medium protects the cell from isoguanine poisoning, presumably by accelerating the excretion of a noxious material from the vacuole. Ascorbic acid also protects the cell from thallium, but in this case it is assumed to act as a chelating agent. Cobaltous chloride poisoning. CoC12 is rapidly concentrated in the yeast cell, and when added to the medium produces a high frequency of respiratory deficient cells. When yeast cells are grown in cobalt, the contents of the nuclear vacuole turn pink, but color does not appear elsewhere in the cell. The persistance of the unoxidized cobaltous ion in the nuclear system eventually results in the death of many cells. Accumulation of fat in the Vacuole. Fat is synthesized in the mitochondria, transported to the nuclear vacuole and metabolized in the nucleolus. When yeast cells are poisoned by (a) Cobalt, (b) propamadine isethionate, or (c) quanylic acid, fat accumulates in the nucleolus, hence these substances in hibit the normal oxidation of fat in the nuclear system. The action of these poisons and the nature of the poisons and the nature of the protection afforded by ascorbic acid is discussed in relation to the organization of the yeast cell in the supplementary sheets of this application.